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Journal of the Acoustical Society of America / Volume 131 / Issue 1 / JASA EXPRESS LETTERS

Acoustic scattering from density and sound speed gradients: Modeling of oceanic pycnoclines

J. Acoust. Soc. Am. Volume 131, Issue 1, pp. EL54-EL60 (2011); (7 pages)

Tetjana Ross1 and Andone C. Lavery2

1Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada tetjana@dal.ca
2Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 alavery@whoi.edu

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A weak-scattering model that allows prediction of acoustic scattering from oceanic pycnoclines (and the accompanying sound speed gradients) based on hydrographic profiles is described. Model predictions, based on profiles from four locations, indicate that scattering from oceanic pycnoclines is measurable using standard scientific sonars operating at frequencies up to 200 kHz but generally only for pycnocline thicknesses less than 10 m. Accurate scattering models are key to assessing whether acoustic remote sensing can be used to map oceanic pycnoclines and for determining whether scattering from pycnoclines needs to be taken into account when estimating, for instance, zooplankton abundance from acoustic data.

© 2012 Acoustical Society of America

Acknowledgments

We thank Jim Moum for the NJ profile and www.nodc.noaa.gov for the others.

Article Outline

  1. Introduction
  2. Weak-scattering model for oceanic pycnoclines
  3. Model predictions for oceanic pycnoclines
    1. Predictions based on hydrographic profiles
    2. Scattering dependence on ΔT and ΔS
    3. Model limitations
  4. Conclusions

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KEYWORDS and PACS

Keywords

acoustic wave scattering, acoustic wave velocity, hydrophones, oceanographic equipment, oceanographic techniques, remote sensing, sonar, underwater sound

PACS

  • 43.30.Gv

    Backscattering, echoes, and reverberation in water due to combinations of boundaries

  • 43.30.Ft

    Volume scattering

ARTICLE DATA

History
Received 31 Aug 2011
Accepted 21 Nov 2011
Published online 20 Dec 2011
Digital Object Identifier
http://dx.doi.org/10.1121/1.3669394

PUBLICATION DATA

ISSN

0001-4966 (print)  

Publisher

$abstract.society.value is a Member of CrossRef Acoustical Society of America

  1. Fisher, F. H., and Squier, E. D. (1975). “Observation of acoustic layering and internal waves with a narrow-beam 87.5-khz echo sounder,” J. Acoust. Soc. Am. 58, 1315–1317JASMAN000058000006001315000001. [ISI]
  2. González-Pola, C., Fernández-Díaz, J. M., and Lavín, A. (2007).“Vertical structure of the upper ocean from profiles fitted to physically consistent functional forms,” Deep-Sea Res. I 54, 1985–2004.
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  5. Lavery, A., and Ross, T. (2007). “Acoustic scattering from double-diffusive microstructure,” J. Acoust. Soc. Am. 122, 1449–1462JASMAN000122000003001449000001.
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Figures (click on thumbnails to view enlargements)

FIG.1
Scattering geometry for (a) the double-diffusive (or very sharp pycnocline) and (b) the general pycnocline scattering model. The dotted lines show the sound emanating from the source/receiver, those that are solid indicate the length of the integration length scale, Le. Horizontal lines are used to suggest the scattering surfaces implicit in the model formulation. The irregularly spaced lines [dark in (a) and light in (b)] illustrate the gradient in acoustic impedance over the interface thickness, Δz. The regularly spaced horizontal lines [light in (a) and dark in (b)] illustrate the regular spacing of the sublayers (Δ) within Le.

FIG.1 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.2
Examples of applying the pycnocline scattering model, Eq. ( 4 ), by fitting the González-Pola et al. (2007)2 model to profiles collected in the Equatorial Pacific (EP), the Scotian shelf (SS), a Nowegian fjord (Nf), and the New Jersey continental shelf (NJ). (a) Temperature profiles (thin lines) and the theoretical fit (thick lines). (b) Same as (a), but for salinity. (c) Density profiles calculated from theoretical profiles shown in (a) and (b). (d) Sound speed profiles calculated from theoretical profiles shown in (a) and (b). (e) Predicted volume scattering strength at 38 kHz as a function of depth for the theoretical profiles shown in (c) and (d). (f) Predicted scattering (scattered pressure) at the pycnocline depth (indicated in the legend) as a function of frequency. Scattering predictions are based on the functional fits evaluated with a vertical resolution of 0.75 mm.

FIG.2 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.3
(Color online) A false color map of predicted pycnocline Sv at 38 kHz as a function of surface T and S. The pycnocline scattering model is applied to density and sound speed profiles calculated from shape parameters based on the New Jersey shelf data. The T and S steps across pycnoclines of fixed depth (30 m) and thickness (Δz ≈10 m) were varied. The hatched area in the bottom left is where the surface water would freeze, and the one on the right is where the density profiles would be unstable.

FIG.3 Download High Resolution Image (.zip file) | Export Figure to PowerPoint



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